A major problem with the treatment of urban runoff water is that the peak flow caused by heavy rain is much larger than the average flow. Since it is too expensive to build a sewage treatment station that can handle both sewage and peak flow runoff water different methods have been used. One method is the treatment of sewage and runoff water together in a sewage treatment station. When the flow rate exceeds the capacity of the station excess flow is flooded untreated via a spillway to a recipient. Another method, described in U.S. Pat. No. 4,298,471, uses a basin consisting of several compartments which are interconnected. The first compartment is connected to a channel through which the runoff water is entered. The sewage treatment station is also connected to the first compartment. The last compartment in the series is connected to the surrounding lake. The idea with this solution is that the urban runoff water resulting from heavy rain successively fills up the compartments thereby pushing lake water back into the lake. The basin thus acts as a buffer for the runoff water. This method thus means a substantial improvement relative to the method described before. However, because of temperature and density differences between the runoff water and the lake water, it cannot be avoided that mixing between the contents of the different compartments and between the last compartment and the surrounding lake occur. A part of the polluted runoff water therefore enters into the surrounding lake.